""For example, there's no point in a 120 dB THD+N figure for a source if the figures for the Onk and ADA are lower.""

Actually, you want a lower noise floor than the Onk or ADA, if you can do it.

Looking at the Cambridge Audio AXC35 CD player, it is spec'ed to -93 dB S/N.

The Yamaha CD-2100 has a spec of -116 dB.

It is not a subtle difference between the 2, and it is doubtful the Amazon unit can match the Cambridge.